The present invention involves an aerosol method for producing silver containing particulate materials, powders including silver-containing particles producible by the method, uses of the powders in the manufacture of electronic and other devices and the d vices so manufactured.
Silver particles are widely used in the manufacture of electrically conductive thick films or electronic devices and for other uses. Examples of thick film applications for silver particles include to make internal electrodes in multi-layer capacitors; interconnections in multi-chip components; conductive line in auto defoggers/deicers, photovoltaic modules, resistors, inductors, antennas and membrane switches; electromagnetic shielding (such as in cellular telephones), thermally conductive films; light reflecting films; and conducting adhesives. A trend continues, however, to produce ever smaller particles for these and other applications. Generally, desirable features in such small particles include a small particle size; a narrow particle size distribution; a dense, spherical particle morphology; and a crystalline grain structure. Existing technology for manufacturing silver-containing particles could, however, be improved with respect to attaining all, or substantially all, of these desired features for particles used in thick film applications.
One method that has been used to make small particles is to precipitate the particles from a liquid medium. Such liquid precipitation techniques are often difficult to control to produce particles with the desired characteristics. It is especially difficult by the liquid precipitation route to obtain particles having dense, spherical particle morphology and with good crystallinity.
Aerosol methods have also been used to make small silver particles. One aerosol method for making small particles is spray pyrolysis, in which an aerosol spray is generated and then converted in a reactor to the desired particles. Spray pyrolysis systems involving silver have, however, been mostly experimental, and unsuitable for commercial particle production. Furthermore, control of particle size distribution is a concern with spray pyrolysis. Also, spray pyrolysis systems are often inefficient in the use of carrier gasses that suspend and carry liquid droplets of the aerosol. This inefficiency is a major consideration for commercial applications of spray pyrolysis systems.
There is a need for improved silver-containing powders, for improved methods of manufacturing silver-containing powders and for improved products incorporating or made using improved silver-containing powders.
The present invention provides high quality, micro-size silver-containing particles of a variety of compositions and particle morphologies, and with carefully controlled particle size and size distribution, and an aerosol method for producing the particles. The particles are useful for making a variety of products.
Through careful control of the aerosol generation, and particularly when used in combination with other features of the present invention, the present invention permits preparation of very high quality powders of silver-containing particles that preferably have a weight average size in a range of from about 0.1 micron to about 4 microns, and for many applications with a range of from about 0.5 micron to about 2 microns in size. The powders have a narrow size distribution such that typically at least about 90 weight percent of the particles are smaller than about twice the weight average size of the particles. Furthermore, typically less than about 10 weight percent, and preferably less than about 5 weight percent, of the particles are smaller than about one-half the weight average size.
The invention includes both single-phase and multi-phase, or composite, particles useful for a variety of product applications, including for use as catalysts and in the preparation of thick film paste formulations, such as are used for depositing silver-containing films during manufacture of various electronic and other products. Multi-phase materials may be in a variety of morphological forms, for example in an intimate mixture of two or more phases or with one phase forming a surface coating over a core including another phase.
One preferred class of multi-phase particles includes a metallic silver-containing phase and a nonmetallic phase, which frequently includes a ceramic material. The nonmetallic phase could be in the form of a coating around a core of the metallic phase, in the form of small domains dispersed in a matrix of the metallic phase, or in some other form. A variety of ceramic materials can be used to effect a variety of beneficial modifications to particle properties, such as a modification of densification/sintering properties for improved compatibility and bonding with ceramic layers used in electronic devices, and especially when the silver-containing powder is used to make internal electrodes in multi-layer capacitors. An important use of these multi-phase particles is to reduce or delay film shrinkage during firing in the manufacture of electronic devices. One preferred group of ceramic materials for use in the multi-phase particles are titanates, as are frequently used in dielectric layers of multi-layer capacitors. Another preferred group of ceramic materials for use in multi-phase particles includes silica, alumina, titania and zirconia.
The silver in the powder, whether in single phase or multi-phase particles, is typically present in a metallic phase. In one embodiment, the particles in the powder include a high quality silver alloy, with one preferred alloy being with palladium. It has been found that the quality of the alloy is highly dependent upon the manner in which the process is carried out and the specific processing conditions. When preparing particles including a silver/palladium alloy, the reactor temperatures should preferably be in the range of from about 900xc2x0 C. to about 1200xc2x0 C., with even narrower temperature ranges being more preferred.
The process of the present invention for making the silver-containing particles involves processing of a high quality aerosol including a silver-containing precursor. The aerosol includes droplets of controlled size suspended in and carried by a carrier gas. In a thermal reactor, typically a furnace reactor, the liquid of the droplets is vaporized, permitting formation of the desired particles in an aerosol state. According to one embodiment of the present invention, an aerosol at a high droplet loading and at a high volumetric flow rate is fed to a reactor, where particles are formed. In addition to the high droplet loading and high volumetric flow rate, the aerosol also includes a narrow size distribution of droplets such that the particles exiting the reactor also have a narrow size distribution, with preferably at least about 75 weight percent, and more preferably at least 90 weight percent, of the particles being smaller than about twice the weight average particle size.
With the process, and accompanying apparatus, of the present invention, it is possible to produce high quality silver-containing powders at a high production rate using spray pyrolysis. This represents a significant advancement relative to the small laboratory-scale processes currently used.
An important aspect of the method of the present invention is aerosol generation, in which a high quality aerosol is produced having a controlled droplet size and narrow droplet size distribution, but at a high volumetric flow rate and with high droplet loading. An ultrasonic generator design is provided for generation of the high quality aerosol at a high generation rate.
The aerosol generation is particularly advantageous for producing aerosol droplets having a weight average size of from about 1 micron to about 5 microns, preferably with no greater than about 30 weight percent of the droplets being larger than about two times the average droplet size.
High quality aerosol production is accomplished also with high droplet loading in the aerosol. The droplet loading is preferably greater than about 5xc3x97106 droplets per cubic centimeter of the aerosol. Furthermore, the aerosol typically includes greater than about 0.083 milliliters of droplets in the aerosol per liter of carrier gas in the aerosol. This high droplet loading is also accomplished at a high aerosol production rate, which is typically at a rate of greater than about 25 milliliters of droplets of liquid feed per hour per ultrasonic transducer. Total aerosol flow rates are typically larger than about 0.5 liter per hour of liquid droplets at the high droplet loading and with the narrow droplet size distribution.
Aerosol generation for particle manufacture of the present invention is believed to represent a significant improvement relative to current powder manufacture operations, which are mainly for experimental purposes. These laboratory-scale processes typically use aerosols at only low rates and normally without a high aerosol density. With the aerosol generator of the present invention, however, high rates of droplet production are possible with efficient use of carrier gas. In one embodiment, the aerosol generator includes a plurality of ultrasonic transducers underlying a reservoir of liquid feed that is ultrasonically energized during operation. The aerosol generator includes multiple gas delivery outlets, or ports, for delivering the carrier gas to different portions of a liquid feed reservoir, so that droplets generated from the different portions of the reservoir are efficiently swept away to form the aerosol. A preferred embodiment includes at least one gas delivery outlet per ultrasonic transducer.
The process and the apparatus of the present invention are also capable of producing silver-containing powder at a high rate without high losses of silver in the system. This is accomplished through careful control of process equipment and operating parameters in a manner to reduce system residence times to inhibit high production losses. An important aspect of the process of the present invention is operation of a pyrolysis furnace at high flow rates and in a manner to reduce the potential for losses of silver. When operating at the high Reynolds numbers typically encountered in the furnace with the present invention, it is important to carefully control the furnace temperature and temperature profile. For example, it is important to operate the furnace so that both the average maximum stream temperature and the maximum furnace wall temperature are low enough to avoid undesirable volatilization of silver or other components.
In a further embodiment aspect of the process and apparatus of the invention, the particles may be advantageously cooled for collection in a manner to reduce potential for silver losses. The particle cooling may advantageously be accomplished with a very short residence time by radial feed of a quench gas into a cooling conduit through which the particle-containing aerosol stream flows. In this manner, a cool gas buffer is developed around the inner walls of the cooling conduit, thereby reducing thermophoretic losses of particles during cooling.
Also, the particle manufacturing process of the present invention is versatile and may be adapted for preparation of a variety of silver-containing particulate materials for a variety of applications. In that regard, one embodiment of the present invention includes concentration of the aerosol by at least a factor of about two, and more preferably by a factor of greater than about five, before introduction of the aerosol into the reactor. In this manner, substantial savings may be obtained through lower heating requirements in the reactor, lower cooling requirements for product streams from the reactor and smaller process equipment requirements.
In another embodiment, the process of the present invention involves classification by size of the droplets in the aerosol prior to introduction into the pyrolysis furnace. Preferably, droplets larger than about three times the average droplet size are removed, and even more preferably droplets larger than about two times the average droplet size are removed.
In yet another embodiment of the present invention, the particles are modified following manufacture, while still dispersed in an aerosol stream, prior to particle collection. In one aspect, the particles may be subjected to a coating following manufacture, such as by chemical vapor deposition or gas-to-particle conversion processes. Preferred coating processes include chemical vapor deposition and physical vapor deposition. In a further aspect, the particle modification may involve a structural modification, such as a post manufacture anneal to improve crystallinity or to alter particle morphology, and without agglomeration of the particles.
The present invention also includes thick film paste formulations including the silver-containing particles of the present invention and processes of manufacturing films from formulations. Also included in the present invention are methods for making electronic and other products using the palladium-containing particles. The present invention provides a variety of products made using powder of the present invention. These products include electronic devices, such as multi-layer capacitors and multi-chip modules, and other products, such as catalysts, electrochemical cells and others. Also, the powders are particularly useful for making high definition patterned circuit lines with a close line spacing.